The disclosures herein relate generally to flexible circuits and more particularly to flexible circuits having vias with predetermined sidewall angles.
Polyimide film is commonly used as a substrate for flexible circuits. Patterning such substrates primarily is accomplished by such process steps as mechanical punching or drilling, laser ablation and chemical etching. Etched features such as vias or throughholes are commonly present in flexible circuits for reasons of interconnection.
WO 00/11066 discloses polyimide films formed from polyamic acids obtained upon the reaction of p-phenylene bis(trimellitic acid monoester anhydride), hydroxydiphthalic dianhydride, p-phenylenediamine and 4,4xe2x80x2-diaminodiphenyl ether in an organic solvent. The film has a high modulus, high elongation and low coefficients of linear expansion and expansion due to moisture absorption.
The sidewalls of the features show different slopes depending on whether physical or chemical methods are used for forming the feature. For example, punching or drilling basically provides parallel sidewalls. Laser ablation can provide specific slopes, however it is extremely expensive to perform. Chemical etching provides features with angled sidewall slopes. For example, the sidewall slope of a popular commercial polyimide, Kapton(trademark), is from about 20-30 degrees when chemically etched, using a photoresist mask. However, it is extremely difficult to control or alter the angle when chemically etching with a photoresist mask due to a variety of issues, including uncontrolled swelling of the polyimide. Surface treatments such as corona and plasma etching have also be used to etch polyimides, and to achieve shallow angles. However, surface treatments have not been successful at producing angles in the polyimide above about 15%.
Depending on the applications, significantly different polyimide via slope angles from that normally obtained by chemical etching would be desirable. For example, a shallow angle may be desirable if more stiffness is needed or if the etch feature must interface with another part. Alternatively, a steep via angle may be desirable if the trace density is high or more flexibility of the circuit is desired.
It has now been discovered that flexible circuits having substrates formed from certain polyimide substrates may be chemically etched by controlled means to form an etched feature having a sidewall of a predetermined slope.
Further, flexible circuits may be formed wherein the substrate has been chemically etched by controlled means such that there are multiple etched features having sidewalls of differing predetermined slopes.
Also, flexible circuits formed from such polyimide substrates do not show the uncontrolled swelling when such an etch is attempted as has been observed with conventional polyimide substrates.
Flexible circuits may also be formed where one etched feature has at least one sidewall having two differing slopes.
The instant invention provides a flexible circuit having a polyimide substrate containing at least one chemically etched feature, wherein such etched feature has sidewalls having a predetermined slope angle.
The instant invention further provides a process for forming a flexible circuit having a polyimide substrate and containing at least one chemically etched feature, wherein such etched feature has sidewalls with a predetermined slope angle.
More specifically, the instant invention provides a flexible circuit having a polyimide substrate comprising at least one chemically etched feature with sidewalls that have a preselected slope angle of from about 15 degrees to about 75 degrees.
The invention further provides a flexible circuit having a polyimide substrate comprising at least a first and a second chemically etched features having one or more sidewalls of predetermined slopes, wherein said first feature has a predetermined slope which differs from the predetermined slope of the second feature. More features may be included in the plurality which have the same or differing predetermined sidewall slopes as the first and second feature.
Flexible circuits may also be formed where one etched feature has a first sidewall and a second sidewall, each sidewall having a different predetermined slope.
The polyimide substrate of flexible circuit of the invention comprises a polyimide formed from at least one polyamic acid having an ester group therein. One useful monomer is p-phenylene bis(trimellitic acid monoester anhydride) monomer (TMHQ). Other useful monomers include 1,3-diphenol bis(anhydro-trimellitate), 1,4-diphenol bis(anhydro-trimellitate), ethylene glycol bis(anhydro-trimellitate), biphenol bis(anhydro-trimellitate), oxy-diphenol bis(anhydro-trimellitate), bis(4-hydroxyphenyl sulfide) bis(anhydro-trimellitate), bis(4-hydroxybenzophenone) bis(anhydro-trimellitate), bis(4-hydroxyphenyl sulfone) bis(anhydro-trimellitate), bis(hydroxyphenoxybenzene), bis(anhydro-trimellitate), 1,3-diphenol bis(aminobenzoate), 1,4-diphenol bis(aminobenzoate), ethylene glycol bis(aminobenzoate), biphenol bis(aminobenzoate), oxy-diphenol bis(aminobenzoate), bis(4 aminobenzoate) bis(aminobenzoate), and the like.
When other types of polyimides are etched, they provide the same slope each time they are etched, and it is not possible to vary the slope, or predetermine a desirable slope. Further, the polyimides conventionally used in electronic packaging are very subject to swelling during etching, which renders control of the etch depth and angle difficult at best. Preferred substrates useful in circuits of the invention do not swell more than about fifty percent (50%) by volume when subjected to chemical etchants commonly used to etch flexible circuits.
The invention also provides a process for forming a flexible circuit having a polyimide substrate containing at least one chemically etched feature thereon, wherein such etched feature has sidewalls with a predetermined slope angle.
As used herein, these terms have the following meanings.
1. The terms xe2x80x9cetched featurexe2x80x9d and xe2x80x9cchemically etched featurexe2x80x9d are synonymous and mean an indentation, via, hole, or the like, which is formed in one or more layers of the flexible circuit by means of etching at least a portion of at least one layer in a chemical bath.
2. The term xe2x80x9csidewallsxe2x80x9d refers to the sides of an indentation or hole feature formed by the etching of one or more layers of the substrate. It should be noted that the number of sidewalls varies with the geometry of the etched feature. For example, a circular etched feature has only one sidewall, which may have one or more slope angles. A square will have four sidewalls, which may each have one or more slope angles.
3. The term xe2x80x9cslope anglexe2x80x9d refers to that angle measured from the bottom of the feature to the top of the etched feature.
All percentages, ratios and amounts herein are by weight unless otherwise specifically noted.